Promise.resolve().then(() => console.log(1)).then(() => console.log(3))
Promise.resolve().then(() => console.log(2))

new Promise((resolve) => {
  console.log(1)
  resolve()
  console.log(2)
}).then(() => console.log(4)).then(() => console.log(5))

/**
 * 单线程是异步产生的原因
 * 事件循环是异步的实现方式
 * 
 * 
 */